Smell of gasoline from inside the cabin of my Nissan N15 Pulsar 1998

You may have an actual fuel leak and this being an older car, that is certainly a consideration. Check the fuel tank, the fuel lines from the tank to the engine, the fuel filter, and the fuel connections on the engine for rust and/or leaks.

The most common cause, however, is a problem with the "EVAP" system. This could be due to a failed gas cap or a saturated charcoal canister. Either check those yourself or have them checked at a shop.

No, an oil leak will not smell like gasoline.

If it's smelling a bit like gasoline it could also be exhaust gases entering the cabin through possibly a leak in the exhaust from rust/corrosion and then through a small leak in the cabin. The reason this may only occur in the morning is because when the engine is cold more fuel is injected than usual which will help the engine heat up quicker and often cars can be more smelly in the morning for this reason.

If it is the case that you have an actual fuel leak, a problem with the evap system as stated in the previous answer or exhaust gases entering the cabin either way they are creating an unsafe and hazardous environment for you to be driving your car in and you should get it inspected professionally asap.

And yes just like the first answer stated oil smell is very distinguishable from petrol but if you have a few smells mixed together it may cause some confusion and all the more reason it will need to have a professional inspection.

Look for the easy answer. I had that problem with an '85 Nissan 300ZX . I just looked under the hood with the engine running and found a wet, not-tight connector in the fuel injector lines . Fix only required that I buy an extra long phillips screwdriver and use it.